This invention relates to variable pitch propellers used on aircraft, and more specifically, to an improved backup governing system for such propellers.
Variable pitch propellers are employed on many differing types of aircraft having power plants ranging from piston engines to gas turbines. Conventionally, such systems include redundant main control systems with one of the control systems being operable to take over the pitch control function if the other main system malfunctions. Moreover, it is common practice to provide a backup governing system for the redundant main control systems, particularly where the main control systems are electronic pitch control (EPC) systems. The backup governing system should be capable of determining when a propeller overspeed condition exists and causing propeller blades to move toward a coarse pitch condition which is sufficient to slow the rate of rotation of the propeller to a maximum allowable speed.
In addition, the backup control system should also be capable of determining when a low pitch condition (also referred to as a xe2x80x9clow pitch stopxe2x80x9d) exists wherein the pitch of the propeller becomes less than that defined as a minimum, in-flight allowable pitch (often called xe2x80x9cflight idlexe2x80x9d pitch) and cause the propeller blades to return to a coarser pitch that is at least equal to or greater than the flight idle pitch.
Still further, if the variable pitch propeller system is one where the pitch of the blades can be changed to cause a reverse thrust condition, as, for example, employed at slowing an aircraft on a runway just after having landed, the backup governing system must include provision for manually disabling the backup functions providing overspeed protection and low pitch stop.
As propeller pitch control systems, including EPC systems conventionally employ hydraulic fluid under pressure as a means for controlling the pitch of the propeller blades, it is highly desirable to provide a backup control system which is operable notwithstanding flight conditions such as a momentary loss of hydraulic power. It is also desired to eliminate mechanical gear trains or other connections between the rotating and stationary parts of the propeller.
Furthermore, an additional constraint is the requirement that the backup governing system interfaced with three existing pressure signals which are available to the propeller from its control. This constraint minimizes cost and enhances the ability to retrofit a system on existing propeller control systems.
The present invention is directed to overcoming one or more of the above problems.
It is the principal object of the invention to provide a new and improved backup governing system for a variable pitch propeller. More specifically, it is an object of the invention to provide such a system wherein the components are carried by the propeller, eliminating a mechanical interface for the backup control between rotating and stationary parts of the propeller system and which may interface with existing pressure signals in a conventional system.
An exemplary embodiment of the invention achieves the foregoing object in a variable pitch propeller system that includes a rotatable propeller hub that is adapted to be driven by a prime mover. A double acting piston is carried by the hub and propeller blades having shanks journaled in the hub are provided. A linkage connects the piston to the shanks so that movement of the piston will cause rotation of the shanks within the hub. A transfer bearing is provided for providing at least first and second streams of hydraulic fluid under pressure. The first stream is adapted to be applied to one side of the piston and the second stream is adapted to be applied to the opposite side of the piston. A main control is provided for regulating the pressures of the first and second streams to set the pitch of the blades and the hub. According to the invention, there is provided a backup governing system carried by the hub and hydraulically interposed between the main control and the piston and which includes a metering valve having a metering valve member movable between a plurality of metering positions, first and second, opposing biasing elements applying oppositely directed forces to the valve member, a flyweight assembly connected to the valve member applying a speed dependent valve positioning force to the valve member in opposition to the first biasing element and in addition to the second biasing element. Also included is an actuator that is operatively associated with the second biasing element and which is operable to change the bias applied by the second biasing element to the valve member. A first stream control valve is connected to the actuator and is interposed between the transfer bearing and the piston and is operable to control the flow of the first stream to the one side of the piston.
In a preferred embodiment, the first stream is a stream that moves the propeller pitch towards a fine pitch condition and the second stream is a stream that moves the propeller towards a coarse pitch position.
In one embodiment of the invention, the actuator includes an actuator piston having a side hydraulically connected to the metering valve. The metering valve is operable to direct the first or fine pitch stream to the actuator piston side when the metering valve member is moved by the flyweight assembly to a predetermined position indicative of an undesirable occurrence in the operation of the system as, for example, an overspeed condition or a low pitch condition.
In one embodiment, a linkage path, from the double acting piston to the flyweight assembly changes the spool of the flyweight assembly from a force control valve to a motion control valve whenever a low pitch stop is required. When this occurs the flyweight assembly simply becomes another link in the path. The link that follows the cam has enough mass so that it always follows the cam, thus, movement of the double acting piston, during a low pitch condition will move the spool valve directly without any spring or flyweight force influencing the motion of the valve.
One embodiment of the invention contemplates that the actuator and the first or fine pitch stream control valve include a piston valve having a piston surface hydraulically connected to the metering valve to receive the first or fine pitch stream when the metering valve is moved to a predetermined position by the flyweight assembly. Also included is a valve surface for halting flow of the first or fine pitch stream to the double acting piston when the metering valve is moved to the predetermined position by the flyweight assembly.
Preferably, the metering valve is a spool valve and the metering valve member is a spool having opposite ends. The first biasing element and the flyweight assembly are connected to one of the spool ends and the second biasing element is connected to the other of the spool ends.
Preferably, the biasing elements are springs.
The invention also contemplates the provision of a reverse enabling valve hydraulically interposed between the actuator piston and the metering valve and operable to prevent the first or fine pitch stream from being applied to the actuator piston side. In this embodiment, the reverse enabling valve may be a hydraulically operated valve responsive to a hydraulic signal in the form of a third stream of hydraulic fluid passing through the transfer bearing.
The invention also contemplates the provision of a hydraulic discharge path in fluid communication with the double acting piston fine pitch side, a flow limiter in the discharge path, and a valve operated bypass about the flow limiter.
The flow limiter may be an orifice and the discharge path operates as a hydraulically operated pitch delay valve.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.